The present disclosure relates to an image forming apparatus utilizing electrophotography such as copiers, printers, and facsimile machines. More particularly, the present disclosure relates to an image forming apparatus of an intermediate transfer type that primarily transfers a toner image formed on an image carrying member to an intermediate transfer belt and that then secondarily transfers the toner image from the intermediate transfer belt to a recording medium.
Conventionally, there is known an image forming apparatus of an intermediate transfer type that includes an endless intermediate transfer belt rotated in a predetermined direction and a plurality of image forming portions provided along the intermediate transfer belt. This image forming apparatus primarily transfers, by sequentially superimposing on one another, toner images of different colors onto the intermediate transfer belt from photosensitive drums (image carrying members) provided in the image forming portions, and then secondarily transfers them to a recording medium.
In a color image forming apparatus of the intermediate transfer type, when an intermediate transfer belt of an elastic material is used, as the number of printed sheets increases, a toner external additive adheres to the surface of the belt and thereby causes it to whiten. The surface friction coefficient on the belt, unwhitened, is high but lowers with the progress of whitening.
The progress of whitening of the surface of the belt causes, during density calibration, variation in the output value of an image density sensor (ID sensor) that senses the density of a reference image formed on the surface of the belt, and leads to lower calibration accuracy. To avoid that, conventionally, the assembly of the image forming apparatus includes an application process in which a toner external additive is previously applied to the surface of the intermediate transfer belt. However, in recent years, improved image density sensors and calibration methods have alleviated the effect of the whitening of the surface of the belt, and thus the application process tends to be omitted to reduce manufacturing steps and cost.
On the other hand, when organic photosensitive members (OPC), which are laid with organic photosensitive layers on their surfaces, are used as photosensitive drums, photosensitive layers are electrostatically charged during the process of preparatory charging or potential adjustment for the photosensitive drums. Owing to this charging, the surface friction coefficient on the unused photosensitive drums before printing is high. That is, when the image forming apparatus is turned on (starts to be used) for the first time, the intermediate transfer belt and the photosensitive drums, both with a high surface friction coefficient, are combined. Here, when driving is started in a state where the intermediate transfer belt is pressed against the photosensitive drums, a high drum-belt friction force is produced. A drum-belt friction torque propagates to the blade edge of cleaning blades that remove toner on the surface of the photosensitive drums, and thus, though not so high as between belt and drum, a friction force is produced also between blade and drum. The charging performance of the organic photosensitive members lowers as a result of the surfaces being rubbed, and this causes a lower potential; thus, lateral streaks appear in a half image at the primary transfer positions and at the blade edge positions.
As a method for reducing the friction between the intermediate transfer belt and the photosensitive drums, an image forming apparatus is known which has an all apart drivable mode in which, when the intermediate transfer belt and the image carrying members are started up to a speed during image formation, and when the intermediate transfer belt is rotated in the direction opposite to its rotation direction during image formation, with the intermediate transfer belt given such a tension as to be drivable, the image carrying members are all kept away from the intermediate transfer belt.